The present invention relates to a circuit for extracorporeal blood circulation. In particular, though not exclusively, the invention can be advantageously used in single-needle dialysis.
Single-needle dialysis consists in a sequence of brief identical operating cycles. Each operating cycle comprises two stages: an arterial stage and a venous stage. In the arterial stage blood is taken from the patient, through a vascular access, and introduced into the extracorporeal circuit. In the venous stage blood, which has been previously stored in the extracorporeal circuit during the arterial stage, is returned to the patient after being purified through the same vascular access. In single-needle dialysis blood is taken from and returned to the patient through only one access element (for instance a needle or a catheter).
An advantage of single-needle dialysis consists indeed in the possibility to use, when necessary, only one central access element, thus reducing vascular trauma and the risk of thrombotic lesions with respect to the use of two-way access elements or of two access elements coupled together.
Similarly, single-needle dialysis involves a reduced fistula trauma and a subsequent longer duration of said fistula.
Another advantage is for the patient the introduction of only one needle instead of two.
Generally, the extracorporeal circuit for single-needle dialysis comprises: a withdrawal line, which conveys blood from the vascular access to a dialyzing filter; a return line, which conveys blood from the dialyzing filter back to the vascular access; and one or more expansion chambers, arranged before and/or after the dialyzing filter, where blood is stored during the arterial stage. The only access element (needle or catheter) is connected to the withdrawal and return lines through a Y-connection.
At least one arterial pump operates on the withdrawal line. In many cases there is also a venous blood pump operating on the return line.
In the course of the following description we shall refer for reasons of clearness and shortness to extracorporeal blood circuits for single-needle dialysis, equipped with at least two expansion chambers (an arterial chamber and a venous chamber) and designed to operate with two pumps (an arterial pump and a venous pump).
Line closing devices (for instance shaped as pliers) act on the circuit and are arranged close to the ways of the Y-connection connected to the withdrawal and return lines. Said closing devices are automatically controlled at the beginning of the arterial stage, so as to open the withdrawal line and close the return line, and conversely at the beginning of the venous stage, so as to open the return line and close the withdrawal line.
The system controlling switching from one stage to another is usually a pressure/pressure system, in which during the arterial stage (or withdrawal stage) the arterial pump (upstream from the dialyzing filter) fills with blood the filter and the expansion chambers, whereas the venous pump (downstream from the filter) is off; when pressure reaches in a given point of the circuit a given value, the arterial pump stops and the venous pump automatically starts operating (beginning of venous stage or return stage) and conveys blood from the filter to the patient; when pressure in the aforesaid given point of the circuit sinks beyond a given lower limit, the venous pump stops and a new operating cycle begins. This switching procedure allows a relatively high haematic flow with respect to other switching systems.
During the arterial stage a given blood volume (corresponding to Vc=volume of blood treated for each cycle) is stored in the dialysis filter, in the venous expansion chamber and in the arterial expansion chamber. During this arterial stage blood pressure in the extracorporeal system rises. In the arterial stage blood filling the expansion chambers compresses the air contained in said chambers.
The volume of blood treated for each cycle, Vc, is together with the average haematic flow one of the parameters indicating the efficiency of a single-needle dialysis. Said volume Vc is substantially proportional to air volumes within the venous and arterial expansion chambers at the end of the venous stage. In order to obtain sufficiently high values related to the volume of blood treated for each cycle, for instance of about at least 40-60 ml, it is therefore necessary to have two expansion chambers having quite a large size.
Said volume Vc can therefore be considered as proportional to blood pressure difference, measured for instance on the venous line, between the end of the arterial stage and the end of the venous stage. In other words, Vc is proportional to p2-p1 where p1 is pressure in the venous expansion chamber at the end of the venous stage, and p2 is pressure in the venous expansion chamber at the end of the arterial stage. Minimum pressure, p1, is generally higher than zero.
In order to achieve high values of Vc it is therefore possible, on a theoretical level, to build up a proper pressure gap p2-p1 between the arterial withdrawal stage and the venous return stage. However, said pressure gap p2-p1 is necessarily limited, generally to values between 100 and 200 mmHg, because too high a pressure gap could result in an undesired back-filtration, particularly in the case of reduced ultrafiltration or ultrafiltration with highly permeable filters.
Said volume Vc is further proportional to the compliance ance of the dialysis filter. It should be observed, however, that commonly used filters have relatively low compliance values, above all capillary filters.
In short, the volume of blood treated pro cycle Vc, and therefore the efficiency of single-needle dialysis, depends first on the shape, the size and the efficiency of the expansion chambers.
It should be reminded that it is possible, on a theoretical level, to use only one expansion chamber. Note that the use of. two expansion chambers instead of one affects the flow through the dialysis filter; if only the venous chamber is present, blood in the dialyzing filter stands still in the venous stage and flows in the arterial stage, so that flow is intermittent; in the same way, if only the arterial chamber is present, blood flows within the filter intermittently, since it stands still in the arterial stage and moves in the venous stage; conversely, using two expansion chambers flow is almost continuous, although normally variable switching from one stage to another one. Thanks to the continuity of the haematic flow, there are no blood staunching stages, thus reducing the danger of blood coagulation phenomena.
The use of two chambers is therefore advisable both in order to obtain a continuous haematic flow, thus avoiding blood staunching, and to obtain a high volume of blood treated for each cycle, even with low values of pressure gap and with low compliance filters.
The present invention refers to an extracorporeal circuit equipped with at least. two expansion chambers; at least an (arterial) chamber is located in a portion of the withdrawal line between the arterial blood pump and the blood treatment unit (dialyzing filter), and at least another (venous) chamber is located in the return line. In case of a circuit equipped also with a venous blood pump, the venous expansion chamber is located in a return line portion between the treatment unit and the venous pump.
It is generally preferable to use two expansion chambers having the same volume, or if different, with the venous chamber larger than the arterial chamber. The total volume of the two expansion chambers is generally not below at least 150 ml.
For a proper use of the expansion chambers, the latter should be almost completely empty of blood (about 5-10 ml of minimum filling pro chamber) at the end of each venous stage: as a matter of fact, the higher the air volume at the end of the venous stage, the higher the volume of blood treated pro cycle Vc.
The extracorporeal circuit for single-needle dialysis as known, carried out according to the precharacterizing portion of claim 1, is normally equipped with a double pump. The pump operating on the withdrawal line (arterial pump) is usually a roll peristaltic rotary pump. The withdrawal line comprises a pump portion designed to be coupled with the peristaltic pump; said pump portion generally consists of a flexible tube having a diameter, both inner and outer, larger than the diameter of conventional conveying tubes, which form the other flexible portions of the line that are not associated to the pump.
The pump portion is generally connected to the rest of the circuit by means of two fittings made of stiff plastic, one for each end of the pump portion, each of which comprises a stiff sleeve, with an inner diameter variation, having a first opening with a larger diameter, into which an end of the pump portion is introduced and glued, and a second opening opposite the first one having a smaller diameter, into which an end of a conventional extracorporeal circulation tube is introduced and glued.
Said structure makes the circuit complicated from the constructive point of view and increases its manufacturing costs; as a matter of fact, beyond positioning two connecting sleeves for a pump portion it is also necessary to carry out several operations involving the introduction and gluing of the tubes within said fittings.
In said known circuit the arterial and venous expansion chambers, one arranged after the arterial pump and the other one arranged before the venous pump, consist of two separate containers made of transparent plastic, each of which, generally having an axial symmetric shape, is equipped on its bottom with two cylindrical connections placed one beside the other, with vertical axis, an inlet and an outlet one. Each container is attached in a removable way, with a joint between two elastic arms, to a container-holder, which can also be attached in a removable way to an area arranged on one side of the dialysis machine.
During the stage of preparation of a single-needle dialysis treatment, the operator should carry out a series of manual operations aiming at connecting operatively the haematic module to the dialysis machine: said operations comprise among other things the assembly of the container-holder to the machine, the attachment with a joint of the expansion chambers to the container-holder, the fluid connection of the withdrawal line and of the return line to the treatment unit (dialyzing filter), the arrangement of the pump portion of the return line around the corresponding venous blood pump, and so on. Said preparatory operations involve relatively long times and a given difficulty for various reasons: firstly due to the fact that the container-holder should be attached to the machine; secondly, due to the low practicality and functionality of said container-holder; thirdly due to the arrangement of the circuit portions between the treatment unit and the expansion chambers, which makes the manual connection of said circuit portions to the expansion chambers quite difficult; eventually due to the difficulty of arranging the venous pump portion around the corresponding pump.